linux-kernel - Re: [PATCHv3 03/16] pps: fix race in PPS

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Message-ID: <20100805110750.GA10965@pc11.op.pod.cz>
Date:	Thu, 5 Aug 2010 13:07:50 +0200
From:	Vitezslav Samel <vitezslav@...el.cz>
To:	Alexander Gordeev <lasaine@....cs.msu.su>
Cc:	linux-kernel@...r.kernel.org,
	"Nikita V. Youshchenko" <yoush@...msu.su>,
	linuxpps@...enneenne.com, Rodolfo Giometti <giometti@...eenne.com>,
	john stultz <johnstul@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Joonwoo Park <joonwpark81@...il.com>
Subject: Re: [PATCHv3 03/16] pps: fix race in PPS_FETCH handler

On Thu, Aug 05, 2010 at 02:19:51PM +0400, Alexander Gordeev wrote:
> Hi Vitezslav,
> 
> В Thu, 5 Aug 2010 07:19:29 +0200
> Vitezslav Samel <vitezslav@...el.cz> пишет:
> 
> > On Thu, Aug 05, 2010 at 01:06:40AM +0400, Alexander Gordeev wrote:
> > > There was a race in PPS_FETCH ioctl handler when several processes want
> > > to obtain PPS data simultaneously using sleeping PPS_FETCH. They all
> > > sleep most of the time in the system call.
> > > With the old approach when the first process waiting on the pps queue
> > > is waken up it makes new system call right away and zeroes pps->go. So
> > > other processes continue to sleep. This is a clear race condition
> > > because of the global 'go' variable.
> > > With the new approach pps->last_ev holds some value increasing at each
> > > PPS event. PPS_FETCH ioctl handler saves current value to the local
> > > variable at the very beginning so it can safely check that there is a
> > > new event by just comparing both variables.
> > > 
> > > Signed-off-by: Alexander Gordeev <lasaine@....cs.msu.su>
> > > ---
> > >  drivers/pps/kapi.c         |    4 ++--
> > >  drivers/pps/pps.c          |   10 +++++++---
> > >  include/linux/pps_kernel.h |    2 +-
> > >  3 files changed, 10 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/drivers/pps/kapi.c b/drivers/pps/kapi.c
> > > index 55f3961..3f89f5e 100644
> > > --- a/drivers/pps/kapi.c
> > > +++ b/drivers/pps/kapi.c
> > > @@ -326,8 +326,8 @@ void pps_event(int source, struct pps_ktime *ts, int event, void *data)
> > >  
> > >  	/* Wake up if captured something */
> > >  	if (captured) {
> > > -		pps->go = ~0;
> > > -		wake_up_interruptible(&pps->queue);
> > > +		pps->last_ev++;
> > > +		wake_up_interruptible_all(&pps->queue);
> > 
> >   What happens if pps->last_ev overflows? Seems to me it would freeze
> > pps.
> 
> Yes, it will freeze the fds (if they don't use timeouts). But in normal
> circumstances, i.e. when pps_event is called twice a second, it will
> overflow after ~68 years of uninterrupted work. Well, it's the same
> kind of problem as an overflow of struct timespec. I thought it's not
> actually a problem. Should I use u64 instead of unsigned int or add a
> runtime check somewhere?

  If we're using 1PPS it's ~68 years, but someone is trying 5PPS now
(it would overflow in ~13.6 years) - what if someone tries e.g. 100PPS?
It's not the same as overflow of struct timespec! I think it deserves
some treatment.

	Cheers,
		Vita
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